ACCA Manual D • 0.08 in. wg Friction800 CFM • 2 TonsWhisper Quiet (<700 FPM)
800 CFM (2 Ton) Return Air Duct & Filter Grille Size
Calculated duct sizing specifications for 800 CFM of conditioned airflow. Sized via the Equal Friction Method and Huebscher equivalent rectangular equation to maintain static pressure and eliminate air rush noise.
Round Duct Size14" RoundSpiral / Snaplock Pipe
Air Velocity748 FPMFeet Per Minute
Cooling Tonnage2 Tons@ 400 CFM / Ton
Register / Grille16x25Face Dimension
Equivalent Rectangular Duct Sizing Schedule
Exact rectangular sheet metal dimensions offering equivalent airflow resistance to a 14" round pipe
| Height (Depth) | Width | Cross Area | Air Velocity | Aspect Ratio | SMACNA Status |
|---|---|---|---|---|---|
| 10" Deep | 20" Wide | 200 sq in | 576 FPM | 2.0:1 | Optimal (<2:1) |
| 12" Deep | 16" Wide | 192 sq in | 600 FPM | 1.3:1 | Optimal (<2:1) |
Air Terminal & Grille Sizing Guidelines
Recommended face velocities to prevent whistling diffuser vanes
Supply Register Specifications
Recommended Grille Size:16x25 or 20x20 Filter Grille (Minimum 400 sq in face area)
Maximum Target Velocity:500–700 FPM at register face
Diffuser Throw:Spread toward outside walls
Return Air Intake Rules
Return Velocity Target:<450 FPM through filter face
Filter Surface Requirement:Minimum 2 sq ft per Ton
Return Friction Target:0.05 – 0.08 in. wg / 100 ft
HVAC Sheet Metal & Installation Directives
- Return air ducts must be designed with lower velocity (<600 FPM) than supply ducts to prevent loud suction noise.
- For 800 CFM, a 10x20 or 12x16 return drop entering the furnace side ensures smooth airflow through a 16x25 filter rack.
Frequently Asked Duct Sizing Questions
What size return air duct do I need for a 2-ton system?
A 2-ton return requires a 14-inch round duct or a 10x20 rectangular return drop, paired with a 16x25 or 20x20 filter grille.